Are the nosebleeds on both sides or just one nostril? Has the vet suggested a nasal scope and flush? Sometimes nosebleeds can be caused by irritants stuck in the nasal passages. With a scope, they can sometimes see what the cause is and a flush can dislodge the material and cure the problem.
Another thought is to have your dog checked for tick-borne disease--some of them, like anaplasmosis and ehrlichiosis, can reduce the number of platelets produced to the point of causing unexplained bleeding. There may not be many ticks in your area (?) but she may have been exposed elsewhere before making her way to you. Since some of the tick-borne diseases aren't easily detected, sometimes a vet will just treat with doxycycline for 4 to 6 weeks and see if symptoms improve.
I hope you come up with some answers soon, Zena. I know how scary canine nosebleeds are.